1. Field of the Invention
The present invention relates to a dual hydraulic brake system for a vehicle, and in particular to a dual hydraulic brake system in which a proportioning valve is positioned between a master cylinder and rear wheel cylinders of the vehicle.
2. Description of the Prior Art
A conventional dual hydraulic brake system is disclosed in Japanese Patent Publication No. 49-15193 issued on Apr. 12, 1974. In the conventional dual hydraulic brake system which is depicted in FIG. 4, when hydraulic pressure supplied to a first inlet port of a proportioning valve from a master cylinder exceeds a set value during the braking operation, a valve head formed on a piston cooperates with a valve seat to open and close a first fluid circuit periodically, thereby proportionally reducing the hydraulic pressure supplied to the rear wheel cylinders.
In order to obtain a fully effective and safe braking operation even in the event of fluid leakage in a fluid circuit which connects the master cylinder and the front wheel cylinders, the pressure supplied to the proportioning valve is transmitted directly to the rear wheel cylinders without being reduced. In other words, when the hydraulic pressure, which is supplied to a second inlet port of the proportioning valve and which is equal to the pressure at each of the front wheel cylinders, drops, a sleeve moves in a rightward direction (as shown in the drawing) against a spring and is brought into engagement with a stepped portion of the piston. Due to the resulting engagement between the sleeve and the stepped portion of the piston, the piston is prevented from moving in a leftward direction (as shown in the drawing), thereby permitting direct transmission of the hydraulic pressure from the first inlet port to the rear wheel cylinders.
However, since the sleeve and the spring are arranged in series in the proportioning valve, the length of the valve in the axial direction must be increased in order to accommodate the spring and the sleeve.